bitbake: data: Ensure emit_func honours vardeps flag

bitbake: data: Ensure emit_func honours vardeps flag

Currently, calling a shell function using exec_func may result in
dependent functions being missing, if the shell parser can't detect
the dependency, even if it was specified with the vardeps flag.

This patch ensures the function looks at the flag and considers it
when deciding which other functions need to be output.

[YOCTO #3561]

(Bitbake rev: a906c94328b040d5076571bd593b57be7ee403a5)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>